The Italian added to goals from Gonzalo Rodriguez and Ryder Matos as Vincenzo Montella's side ensured an impressive start to its Europa League campaign.
The Italian struck in the final 15 minutes after Gonzalo Rodriguez and Ryder Matos had put la Viola in control against the Portuguese visitors, who failed to unsettle the hosts throughout the encounter.
With the injured Mario Gomez watching from the stands, Fiorentina struggled to create many meaningful chances in the early exchanges, with Rossi and Gonzalo producing the early efforts of note early on.
Indeed, it was the visitors who came closest in the opening half-hour, with Manchester United loanee Bebe drawing a firm save from Neto.
Right on the 30-minute mark, however, Fiorentina broke the deadlock, with Rossi's devilish in-swinging corner meeting the head of Rodriguez, who glanced his effort past Matias Degra.
The tie remained in the balance after the break as Bebe and Irobiso came closest to leveling matters, but substitute Ryder Matos doubled the advantage just seconds after entering the field as he turned home Borja Valero's cross at the near post.
Vincenzo Montella's side began to slip into cruise control and Rossi soon made certain of the points, picking his spot superbly from inside the area to put a seal on another impressive showing.
The win moves Fiorentina to the top of Group E after the first round of fixtures.